A mapping from strings ("print names") to Symbols.
Namespaces are normally named and can be accessed from a global table.
They correspond to Common Lisp "packages" (which are implemented
using gnu.kawa.lispexpr.LispPackage,
which extends Namespace).
A Namespace is a "weak" mapping in the sense that a
Symbol can be garbage collected even though it is
referenced from a Namespace.